In automobile and other engines, it is common practice to instal a three-way catalytic converter in an exhaust passage to transform nitric oxides (NOx), hydro carbons (HC) and carbon monoxide (CO), which are toxic components of exhaust gas, into non-toxic components.
Three-way catalytic converters do not become active until the temperature has risen to a predetermined temperature, and do not perform correctly until they become active. Until the three-way catalyst has become active, therefore, it may occur that exhaust is discharged into the atmosphere without completely removing toxic components.
One method of activating the three-way catalyst at an early stage is to make the air-fuel ratio of the gas mixture aspirated into the engine leaner than the theoretical (stoichiometric) air-fuel ratio to increase the oxygen concentration in the exhaust, thereby promoting oxidation reactions in the converter so as to raise the catalyst temperature. By making the air-fuel ratio leaner, the discharge amount of HC is also decreased.
In this case, the fuel supply amount is often controlled so that the air-fuel ratio is lean until a predetermined time has passed after engine startup, or until the engine cooling water temperature has reached a predetermined value.
However, the time taken for the catalyst to become active or the cooling water temperature when the catalyst does become active are not constant due to differences of engine temperature or ambient conditions during engine startup. These differences thus tend to make the time for which the lean air-fuel ratio is applied too long or too short. When the applied time is too short, catalyst activation is delayed, and when the applied time is too long, the converter temperature rises too high leading to an increase of NOx discharge and early deterioration of the catalyst. Tokkai Sho 60-230532 published by the Japanese Patent Office in 1985 suggests making the air-fuel ratio leaner during engine warmup after the cooling water temperature has reached a predetermined level, and changing this lean air-fuel ratio according to the cooling water temperature.
However, in the case of this control device, lean control is performed only after the cooling water temperature has reached a predetermined value, and the air-fuel ratio is enriched from startup to when the cooling water temperature reaches the predetermined value. The object of this prior art control device is to maintain engine running performance during startup and to decrease the fuel consumption during engine warmup. This type of control does, however, not necessarily lead to the catalyst becoming active at an early stage.